1. Field of the Invention
This invention is related to a composition of matter having increased resistance to environmental stress corrosion cracking, and more particularly to a composition of matter having a heat-treated ferrous metal matrix with a critical amount of a metal oxide dispersed therein to provide increased resistance to environmental stress corrosion cracking.
This invention is further related to a powder metallurgy article of manufacture having a heat-treated ferrous metal matrix and a critical amount of metal oxide dispersed therein to provide increased resistance to environmental stress corrosion cracking, and a process of producing said article of manufacture.
2. Prior Art
It is well known that ultra-high strength steel is defined as steel with a yield strength in excess of 140,000 pounds per square inch. When heat-treated to a yield strength in excess of 160,000 pounds per square inch, such steel becomes susceptible to environmental stress corrosion cracking.
Environmental stress corrosion cracking is a brittle failure which can result from a combination of stress and exposure to a corrosive environment. In the case of heat-treated ultra-high strength steel, environmental stress corrosion cracking can occur in naturally occurring environments such as sea water or the atmosphere.
Environmental stress corrosion cracking is believed to be a form of hydrogen embrittlement cracking that takes place because the conditions which develop within pits, crevices, and cracks lead to an evolution of hydrogen and its absorption by the steel. A key factor leading to the entry of hydrogen is the condition of low pH or acidity which has been observed to develop within stress corrosion cracks.
It is also well known in the art that non-metallic inclusions in the steel can act as initiation sites for environmental stress corrosion cracking and therefore, such inclusions are detrimental to the steel's properties.
Because of the possibility of environmental stress corrosion cracking, the usefulness of ultra-high strength steels is limited. Therefore, there is a need for a heat-treated ultra-high strength steel which exhibits improved resistance to environmental stress corrosion cracking.